In recent years, synthetic latex has been developed rapidly. Latex products mainly include styrene butadiene latex, neoprene latex, NBRL latex, and acrylic ester latex. Synthetic latex is usually fabricated through emulsion polymerization with water as the medium. This method represents a tendency that solvent-borne products are being converted into aqueous products and has been widely applied to many fields such as rubber, coating materials, and adhesives.
Isoprene is an important conjugated diene mainly used as the monomer for the synthesis of polyisoprene rubber. There are many methods for fabricating isoprene in industrial production, such as olefin aldehyde synthesis, isopentane and isoamylene dehydrogenation, separate of C5 fraction generated during the refining of ethylene, and the acetyenic ketone method, among them the C5 fraction separation method is developed rapidly. In industrial production, isoprene monomer is mainly used to make polyisoprene through solution catalysis or ion polymerization, which can replace natural rubber. Polyisoprene can also be used to prepare emulsion for latex products.
There are two methods for the fabrication of polyisoprene emulsion at present: one is the external emulsification method, i.e. synthesizing cis-1,4-polyisoprene solution through anion polymerization or coordination polymerization with organic solvent as the medium, then high-speed blending the polyisoprene solution, water, and a proper amount of emulsifying agent for emulsification, and finally removing the solvent and concentrating the mixture to obtain polyisoprene emulsion. Polyisoprene latex that can be fabricated with this method includes butyl latex (isoprene-isobutene rubber latex), isoprene latex, and isoprene-acrylonitrile latex.
CN 1324874A discloses a method to obtain a polyisoprene rubber with high cis-1,4 double bond content. The polyisoprene rubber is obtained by polymerization of monomer isoprene, rare metal organic phosphate, and alkyl aluminum in inert organic hydrocarbon solvent.
CN 85102250B discloses a method for fabricating polyisoprene rubber through bulk polymerization under the effect of rare earth catalyst.
U.S. Pat. No. 3,285,869 discloses a method for fabricating polyisoprene emulsion, i.e. using Ziegler-Nata catalyst or Li catalyst to obtain polyisoprene solution through solution polymerization, and then emulsifying and dispersing the emulsion with emulsifying agent before removing the solvent.
U.S. Pat. No. 3,971,746 discloses a carboxylated polyisoprene emulsion, wherein 0.03%˜20% carboxyl is introduced during solution polymerization to improve the stability after emulsification.
The other method is to use the traditional emulsion polymerization process and water as the medium to obtain a emulsion by free-radical copolymerization of isoprene and other unsaturated alkene monomers.
WO03/080722 A1 discloses a conjugated diene copolymer emulsion that can be used for dip forming, wherein the conjugated dienes include 1,3-butadiene and isoprene. The copolymer emulsion, obtained through the copolymerization of 30%˜90% of isoprene, 0.1%˜20% of unsaturated carboxylic acid, and 0˜69.9% of other unsaturated monomerscan meet the requirements of dip forming. Dip-formed gloves have excellent physical and mechanical properties and can be used for various purposes.
U.S. Pat. No. 6,878,766B2 discloses a copolymer emulsion for dip forming, wherein the monomers include isoprene. The embodiment is to copolymerize 60%˜81% of butadiene, 2%˜3.5% of unsaturated carboxylic acid, and 17-35% alkylene unsaturated nitrile. Copolymer emulsion obtained can be used for dip forming of gloves.
Polyisoprene emulsion obtained by the first method mentioned above has excellent overall performance. The isoprene has a highly orderly structure. But a great amount of organic solvent is used during the production process which leads to environmental pollution problems and very complicated removal and recycling processes of the organic solvent, and thus to increase cost. In the second method, polar monomers such as acrylonitrile and acrylic acid are often introduced to copolymerize with isoprene in order to increase the strength and stability of the emulsion. But due to the differences in water-solubility and reactivity between comonomers, it is hard to control the polymerization reaction and the levels of molecular branching gelatinization are high. Latex products made by this method is inferior to those made by the first method. Up till now, there is no polyisoprene copolymer emulsion commercialized.
Natural latex gloves have excellent overall performance, but they are easy to cause skin allergies once used on a long-term basis. Synthetic latex products such as neoprene, butyronitrile and butylbenzene will not cause skin allergies due to proteins, and they have strong resistance to grease, chemicals, oxidation, ozonization, low permeability, and better anti-penetration performance than natural latex products. And they have the same hand touch as natural latex products. Neoprene and NBRL latex products have been industrialized, but the recycling treatment of neoprene latex products and the irritating smell of NBRL latex and its products have held back their popularization.